> The -Wa,--gstabs introduces one slight problem.  You want to do it if you get
> a .s file from the native back end, but you probably don't want to do that if
> the  .s  file  comes from another source.  (I.e., I could have my own library
> routines written in assembler.)

Correct.  Although, I doubt many people would use MLton as a compiler for
their assembly libraries.

> What kinds of stabs information does the -Wa,--gstabs actually put in?   Line
> number stuff?  Local symbols?  Any thing else?

Certainly line number stuff.  That's what I've used it for in debugging.
I don't know if there is anything else specifically added by the --gstabs
option.